Capacitive sense elements may be used to replace mechanical buttons, knobs and other similar mechanical user interface controls. The use of a capacitive sense element allows for the elimination of complicated mechanical switches and buttons, providing reliable operation under harsh conditions. In addition, capacitive sense elements are widely used in modern customer applications, providing new user interface options in existing products. Capacitive sense elements can range from a single button to a large number arranged in the form of a capacitive sense array for a touch-sensing surface.
Transparent touch screens that utilize capacitive sense arrays are ubiquitous in today's industrial and consumer markets. They can be found on cellular phones, GPS devices, set-top boxes, cameras, computer screens, MP3 players, digital tablets, and the like. The capacitive sense arrays work by measuring the capacitance of a capacitive sense element, and looking for a delta in capacitance indicating a touch or presence of a conductive object. When a conductive object (e.g., a finger, hand, or other object) comes into contact or close proximity with a capacitive sense element, the capacitance changes and the conductive object is detected. The capacitance changes of the capacitive touch sense elements can be measured by an electrical circuit. The electrical circuit converts the measured capacitances of the capacitive sense elements into digital values.
Because capacitance also changes due to environmental conditions (e.g. changes in humidity and temperature), conventional capacitive sensing algorithms typically use a difference between an immediate value and a filtered baseline average value (initialized by a measurement taken with a non-touch state) to be able to distinguish the presence of a conductive object from an environmental change.
Some electronic devices (e.g., printers, set-top cable boxes, blue-ray players) with mechanical buttons use the state of the button as power is applied (i.e., at power up) to enter special diagnostic modes. Certain set-top cable boxes used by various cable television providers in North America require a service technician to place the set-top box into a diagnostic mode (e.g., a test mode, debug mode) by pressing one or more mechanical buttons on the device while simultaneously applying power to the device.
Although it is desirable to equip set-top boxes with capacitive sense elements as replacements for the mechanical buttons, to do so requires the device to detect the initial presence of a conductive object proximate to one or more capacitive sense elements at power up (i.e., during the initialization process of the device). However, devices using the conventional capacitive sensing algorithms based on filtered baseline averages would fail to detect the presence of a conductive object during the initialization process because the initial presence of the conductive object would prevent an accurate measurement with a non-touch state of the device (e.g., the set-top cable box described above).